This invention relates to a variable frequency self-oscillating half-bridge drive architecture, and, more particularly to a drive architecture for electric loads, such as light sources and the like, that include first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified power supply network for the light source.
A halogen lamp or fluorescent lamp can be driven by an electronic circuit capable of generating signals at a very high frequency compared to the frequency of the power supply network. In particular, frequencies in the 30 to 50 kHz range can be generated compared to the 50-60 Hz of the power supply network.
In this way, the quality of the emitted light and the efficiency of the emitting source can be improved substantially.
This amplified frequency is usually obtained by interposing, between the power supply network and the light emitting source or lamp, a circuit effective to perform a first conversion from AC voltage [50/60 Hz] to essentially DC voltage, with only a limited oscillation or ripple. A subsequent conversion from DC voltage to AC voltage brings the signal up to a higher frequency [30-50 kHz], as shown schematically in FIG. 1.
In particular, FIG. 1 shows a drive circuit 1 which comprises first 3 and second 4 stages cascade connected with each other between a supply network terminal TR and a light source 2.
The AC voltage is rectified and filtered through the first stage 3 to produce a DC voltage which is input to the second stage 4 for conversion to a suitable high-frequency AC voltage for driving the source 2.
In actual practice, there exist several ways of obtaining this conversion from low-frequency to high-frequency AC voltage. In general, two switches SW1 and SW2 are used, suitably driven and connected into a half-bridge configuration, and will be discussed with reference to FIGS. 2A-2D.
More particularly, the switches SW1 and SW2 are connected in series with each other between the terminals T1xe2x80x2 and T2xe2x80x2 of the rectified supply network, which terminals are connected together by a series of a first Cxe2x80x2 and a second Cxe2x80x3 capacitor. The second terminal T2xe2x80x2 of the rectified supply network is connected to a voltage reference, such as a signal ground GND.
The halogen or fluorescent source 2 is placed between a first interconnection node of the switches SW1, SW2 and a second interconnection node of the capacitors Cxe2x80x2, Cxe2x80x3, it being connected in series with a winding or the primary winding of a transformer 4.
Lately the trend among manufacturers of halogen or fluorescent apparatus has been toward increasingly smaller and low-cost designs. Accordingly, a primary concern has become the design of circuits which can be driven using a minimum of components, while being reliable and inexpensive.
In this framing, different design circuits are currently available for driving such apparatus, as shown schematically in FIGS. 2A to 2D.
FIG. 2A shows a conventional drive architecture 1A which comprises an integrated circuit 5 arranged to drive both switches SW1 and SW2 directly.
This prior architecture is quite effective to minimize the number of on-board components, but is highly expensive on account of the high cost of the integrated circuit, and disallows feedback between the working state of the lamp and an oscillator contained in the integrated circuit 5; the oscillator operates, therefore, at a fixed frequency regardless of the operating phase of the light source 3.
A second conventional design is shown schematically in FIG. 2B, wherein a drive architecture 1B drives the switches SW1 and SW2 with the intermediary of two L-C oscillators 6 and 7 which are connected in parallel with the switches SW1, SW2 and triggered by first 8 and second 9 secondary windings wound around the same core of transformer 4.
The drive architecture 1B includes a DIAC circuit connected to the input of the second switch SW2, and an internal circuit node X which is formed between a resistor R and a capacitor C connected in series with each other between the terminals T1xe2x80x2 and T2xe2x80x2of the rectified supply network.
The drive architecture 1B also includes a diode D, connected between the node X and the intermediate node of the switches SW1 and SW2.
It should be noted that the DIAC circuit and diode D are only useful at startup of the drive architecture because, afterwards, the oscillations of the oscillators 6, 7 support themselves automatically.
A prior art modification of the drive architecture 1B is shown in FIG. 2C, generally at 1C in schematic form, and comprises a single oscillator 10 having a respective trigger secondary winding 11. The drive architecture 1C further comprises a driver block 12 connected to the second terminal T2xe2x80x2 of the rectified supply network, and connected to the second switch SW2 directly and the first switch SW1 via a voltage shifter 13.
FIG. 2D shows another state-of-art drive architecture 1D which is widely used because of its low cost. The drive architecture 1D comprises first 14 and second 15 drive circuits connected to the inputs of the switches SW1, SW2 and triggered by first 16 and second 17 secondary windings which are connected to a saturated-core transformer 18, itself connected to the light source 3 by a winding 19.
The frequency of oscillation of the drive architecture 1D is set by the saturated-core transformer 18, which is incapable, however, of ensuring ready repeatability of its characteristics. To achieve stable operation of this transformer, its ferrite components must be carefully selected.
In general, working frequencies are obtained, however, which differ between devices, resulting in the lamp being supplied different power levels.
There has yet to be developed a drive architecture that has adequate structural and functional features to overcome the drawbacks of conventional architectures.
Embodiments of this invention have an oscillation generated within the drive architecture using a trigger winding, rather than by a true oscillator.
Presented, therefore, is a drive architecture for electric loads, in particular light sources and the like, that includes first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified electric power supply network. Each drive circuit block has a respective secondary winding of a transformer associated therewith, and each drive circuit block includes at least a power device and a control circuit portion for controlling the power device. In each control circuit portion of each drive is a circuit block being subjected to a trigger action directly by its associated secondary winding to generate a high-frequency AC current for driving the light source.
The features and advantages of the architecture according to embodiments of the invention will be apparent from the following description of one of the embodiments thereof, given by way of non-limitative example with reference to the accompanying drawings. Although this description covers an architecture adapted to drive light sources, e.g. halogen or fluorescent lamps the invention is not limited to this exclusively, and the description covers this field only for convenience of illustration.